The present invention relates to a structural connection system specially suited for wood frame construction to resist uplift loads created by wind or earthquake forces acting on the structure.
The state of the art and the practice of engineering today provides an accepted method of analysis to determine the lateral forces created and their distribution within wood frame structures due to earthquakes and wind loads. See the chapter entitled "Design Lateral Forces" at pages 173-195 of the Western Woods Use Book, published by Western Woods Products Association, .RTM.1973. Common practice provides for lateral force-resisting elements, called shear walls, in the structures. Shear walls, which may or may not be vertically aligned, are standard wood framed walls sheathed with plywood. The plywood is of a particular grade and species and is nailed in a specific pattern with certain sized nails to achieve the desired strength. Wind loads, earthquakes and other horizontal forces exerted on such a building are transferred through the horizontal floor framing elements (including floor sheathing, joists, etc.) to shear walls extending generally parallel to the direction of the force. These loads introduce vertical forces in the shear walls called uplift loads. These uplift loads create a tendency for the shear walls to lift or move vertically upwardly at one end of the shear wall. The uplift loads tend to lift the shear walls away from the foundation and away from any underlying shear wall.
In this application a structural framing element will often be referred to simply as a stud for ease of reference. However, use of such term is not limiting in any sense.
To resist this uplift load a conventional practice has been to secure the lower ends of the structural framing elements, typically studs and posts, at the shear wall ends (and/or along the length of the shear wall if desired) to the structural framing elements of the next lower level (or directly to the foundation for the bottom level). This has been accomplished using hold-down devices such as are described in U.S. Pat. No. 4,192,118 and U.S. Design Pat. No. D-224,083. Conventional hold-down devices commonly use L-shaped brackets having bolt holes in each leg. The L-shaped bracket is secured to the lower end of a stud (or other structural framing element) by bolts which pass through horizontal holes in the stud. Another bracket is mounted to the upper end of a stud in the underlying level in a similar manner. The two brackets are secured to one another by a rod passing through the floor framing space between the levels. This permits uplift loads to be transferred from one level to the underlying level.
This type of system, although helpful, may not be totally satisfactory in many circumstances. The uplift loads transferred between levels are applied to the ends of the studs at the horizontal holes. The stress concentration at these points creates the potential for the bolts of the hold-down devices to pull out of the ends of the studs. This is especially true at the lower levels where the uplift loads from the overlying levels are cumulatively applied to the ends of the studs. Also, the need to drill horizontal holes in the studs often creates installation problems due to the tight spacing which often exists. Because of the method by which the L-brackets are secured to the studs, play can exist in the connections which reduces the rigidity, and thus the effectiveness, of the system. Another disadvantage is that conventional hold-down devices require quite accurate placement of the brackets and holes, which is difficult to accomplish at the construction site.